PHOTOMETRY 



PHRENOLOGY 



155 



Photometry (Gr. phos, 'light,' and metron, 

 'measure'), the art of measuring the intensity of 

 a source of light, by comparison with a standard of 

 reference (see GAS-LIGHTING). The principles 

 involved in the usual instruments are : ( 1 ) Lam- 

 bert ( 1760), Rumford : equality of shadows cast by 

 two sources at different distances ; when the 

 shadows are equal the intensities are proportional 

 to the squares of the distances. (2) "Equality of 

 illumination through slits in screens ; distances and 

 intensities as before. (3) Bouguer, Ritchie, Leon- 

 hiird Welter : reflection of light from two sources, 

 so that they travel side by side to the eye, the dis- 

 tances being adjusted until they appear equal ; 

 calculation as before. (4) Wheatstone, the same; 

 but the two reflections are from a polished sphere, 

 which is set in motion, so that the comparison is 

 between two looped luminous curves, produced 

 through the persistence of vision. (5) Bunsen : a 

 grease-spot on paper, equally illuminated on l>oth 

 sides, disappears. (6) Ifcihinet : light from one is 

 l>larised by reflection, from the other by refrac- 

 tion ; both pencils are sent through a double 

 rotating quartz, and looked at through a double- 

 refracting prism ; they give coloured images, and 

 the distances are adjusted until the images, on 

 over-lapping, give a white tield. All these methods 

 are unsatisfactory when the sources of light are of 

 different colours, as e.g. a candle and an arc-lamp. 

 Instruments have accordingly l>een devised for 

 applying the above methods to each part of the 

 spectrum <>( the light from each source. The 

 degree of sensitiveness of the eye of the olwerver, 

 or a difference of sensitiveness letween his two 

 eyes, affects the result. In other instruments used 

 as photometers what is measured is not the lumin- 

 ous intensity so much as the radiation : among 

 these we may mention the Radiometer (q.v.); 

 Leslie's photometer, which is a differential thermo- 

 meter ; Bunsen and Roscoe's which measures the 

 quantity of hydrochloric acid formed in a given 

 time from chlorine and hydrogen ; Leon's, which 

 measures the amount of nitrogen lilterated from 

 iodide of nitrogen ; and various instruments based 

 on photographic reactions, which truly measure not 

 the luminous, but the actinic intensity. Stellar 

 photometry is generally contrived by stopping off 

 more or less of the surface of the object-glass, or by 

 polarising apparatus, so as to bring the apparent 

 !>rightness of a star down to that of a standard of 

 Comparison. The usual photometric standards are 

 i 1 ) the English standard candle (see UAS-I.IGHT- 

 INIJ) ; (2) the Hefner- Alteneck amyl-acetate lamp, 

 which has now displaced the candle in Germany ; 

 (3) the carcel, a standard colza-oil lamp, used in 

 France; (4) the Electrical Standards Committee's 

 unit, the light given off by one square centimetre 

 of platinum at its fusing-point ; and (5), in scientilic 

 work, an incandescent electric-lamp under stated 

 iidit.jons of resistance and current, maintained 

 con-tan t. 



Pliotophoite is the name of a comparatively 

 simple apparatus which may be said to achieve the 

 feat of transmitting articulate speech to a distance 

 along a beam of light. It was first described in 

 1SHO by Professor Graham Bell, known in connec- 

 tion with the telephone, at the Boston meeting of 

 tin 1 American Association ; but already in 1878 its 

 inventor had announced the possibility of ' hearing 

 a shadow ' by means of a similar agency. The suc- 

 cess of the photophone depends on the peculiarities 

 of the metal selenium. Crystalline selenium offers 

 a high degree of resistance to the passage of an 

 electric cniu'iit: it is eminently sensitive to ligjit ; 

 and the resistance is less when exposed to light 

 than in the dark, being in some cases only a 

 tin "cut h in the light of what it is in the dark. 



Founding on these peculiarities, Professor Graham 



Bell, his friends, and assistants devised some fifty 

 forms of apparatus, for so varying the transmission 

 of light to prepared selenium as to produce audible 

 sound. In the photophone found most serviceable 

 the transmitter is a plane mirror of silvered micro- 

 scope glass or thin mica ; the receiver, lixed at a 

 distance without any connection, is a parabolic 

 reflecting mirror, in the focus of which is placed a 

 sensitive selenium ' cell,' connected in local circuit 

 with a battery and telephone. When the appara- 

 tus is used, a strong l>eam of light is concentrated 

 by a lens in the plane mirror ; the speaker directs 

 his voice against the back of this mirror, which is 

 thrown into vibrations corresponding with those of 

 the voice. The reflected beam of light, to which 

 corresponding vibrations are also communicated, is 

 directed through a lens to the receiving mirror, 

 and creates in the selenium cell a rapidly variable 

 current, which at the end of the telephone attached 

 liecomes audible again as vocal sound. When first 

 descrilied, the photophone had been used effectively 

 with a distance of 230 yards (over a furlong) be- 

 tween transmitter and receiver. The rays of thy 

 oxyhydrogen light, or of an ordinary kerosene lamp, 

 suffice for transmitting articulate speech. The 

 loudest sounds obtained from the photophone were 

 produced by means of a perforated disc, noiselessly 

 revolving so as rapidly to interrupt the light in 

 transmission. 



It was also found that a very audible sound could 

 be procured from the selenium without the aid of 

 telephone and battery. A beam of intermittent 

 light will produce a strong musical note from the 

 seleniuni. Further experiment showed that sel- 

 enium is not the only substance thus sensitive to 

 light. Still louder sounds than these obtained from 

 the selenium directly, though not articulate, were 

 got from diaphragms of hard india-rubber and of 

 antimony ; and sounds of varying intensity were 

 given out by many other substances, including 

 gold, silver, platinum, copper, zinc, lead, paper, 

 parchment, and wood. 



Photosphere. See SUN. 



Phrenology (Gr., 'a discourse on the mind') 

 was the name given almiit 1815 to the theory of 

 mentai philosophy founded on the observation and 

 discovery of the functions of the brain concerned 

 in intellectual and emotional phenomena, as formu- 

 lated by Gall (q.v.). In Britain the system was 

 amply expounded by Gall's pupil Spur/hciin (q.v.), 

 bv George and Andrew Combe (q.v.), and by Dr 

 ('.Hint -on, founder and first president of the Phreno- 

 logical Society ; and in America by Dr Charles 

 Caldwell of Kentucky, the brothers Fowler, and S. 

 R. Wells. The connection of mind and brain wax 

 an old theory, and a kind of localisation of mental 

 function in the brain had at various times been 

 attempted from thediiys of Albertus Magnus down- 

 wards ; but the first full-fledged system of empirical 

 cranioscopy or phrenology must l>e attributed to 

 Gall, who established his scheme by inspecting the 

 exterior of the heads of a considerable number of 

 persons of different characters ; and believing 

 himself to find portions of the scalp corresponding 

 to their mental and moral faculties, marked 

 out on a model of the head the areas which were 

 prominent in proportion to the strength of the vari- 

 ous faculties. These faculties were by Spurzheim 

 divided into two orders Feelings and Intellect, 

 or Affective and Intellectual Faculties. The Feel- 

 ings were divided into two genera the Propensities 

 and the Sentiments; while the Intellectual em- 

 braced the Perceptive or Knowing and the Reflective 

 I'lii'iilties. Various modifications of Gall's scheme 

 of localising the faculties were made by his pupils. 

 American phrenologists (the Fowlers and S. K. 

 Wells) increased the number of the faculties to 



